In 1983, Kodak introduced its DX system of encoding electrically readable information on a conventional film cartridge containing a 35 mm filmstrip. A DX cartridge has the ISO speed (exposure index), the number of exposures and the exposure latitude (range) of the filmstrip encoded by imprinting a checker board pattern of conductive and insulating patches or sensing areas on the exterior side of the metal housing of the cartridge. There are twelve patches, divided into two rows of six each. An array of electrical probes in a camera's cartridge-receiving chamber contacts the patches to make use of the information when the cartridge is loaded in the chamber. There are two probes for each patch. The purpose of this redundancy is to assure good electrical reliability. Patches 1 and 7 are common/ground. Patches 2 through 6 encode the ISO speed from 25 to 5000 in twenty-four steps. Patches 8 through 10 encode the number of exposures, from 12 to 72 exposures, in seven steps. Patches 11 and 12 encode the exposure latitude, from + or -0.5, EV to +3, -1 EV, in four steps.
After the filmstrip is exposed and returned to the film cartridge, the cartridge is removed from the camera's cartridge-receiving chamber. In order to provide the photographer with a warning that the cartridge now contains exposed film, an exposed film indicator on the cartridge is uncovered. For example, U.S. Pat. No. 5,255,039 issued Oct. 19, 1993 discloses a cartridge having a pull-strip with an exposed film indicator that is originally hidden beneath a cover superimposed over a metal housing of the cartridge. When the pull-strip is manually pulled, the exposed film indicator is moved from beneath the cover into a cut-out in the cover to permit the exposed film indicator to be seen.